The present invention relates broadly to a mixer apparatus and in particular to a monolithic single and double sideband mixer apparatus.
It is well known that a mixer is a device with two or more signal inputs which are usually adjustable, and one common output. The mixer stage in a superheterodyne receiver combines the incoming modulated rf signal with the signal of a local rf oscillator to produce a modulated i-f signal. Crystal diodes are generally used as mixers in radar and other microwave equipment.
Many microwave radar superheterodyne receivers do not employ an RF amplifier. They simply use the crystal-mixer stage as the receiver front end. The noise figure of good crystal-mixer receivers is approximately 7 to 10 db over the range of radar frequencies. This is high compared with low-noise RF amplifiers. However, the noise figure of a crystal mixer is acceptable for many radar applications, especially where simplicity is an important consideration.
The purpose of the mixer portion of the superheterodyne receiver is to convert RF energy to IF energy. A crystal diode with nonlinear resistance characteristic is commonly used as the mixing element. A crystal mixer is broadband when the signal and image frequencies are both terminated in a matched load. The energy impressed in the RF signal channel of a broadband mixer is converted in equal portions to the IF signal and the image. Therefore the theoretical broadband conversion loss can never be less than 3 db. Shortcircuiting or open-circuiting the image termination results in a narrowband mixer. The conversion loss is less in the narrowband than in the broadband mixer.
There is considerable interest in high-performance mixers and receivers for the microwave, millimeter, and submillimeter-wave regions which will also be rugged, reliable, and can be mass produced at low cost. Applications range from radio astronomy to large military imaging systems. Since the packaging of existing high-performance mixers using whisker-contacted Schottky-barrier diodes is quite labor intensive, they are expensive and time consuming to produce. At frequencies above 100 GHz, conventional waveguide mixer circuits become increasingly difficult to make, losses increase rapidly, and circuit elements are located at electrically long distances from the diode leading to large and uncontrolled parasitic elements. Monolithic integration allows circuit elements to be located electrically close to the Schottky diode so that circuit losses are reduced and parasitic elements can be controlled. Moreover, novel coupling and impedance-matching configurations are made possible by using the precision of photolithographically defined circuit elements. A monolithic mixer that is easy to assemble, has the potential for low-cost mass production.
In an application of interest entitled "Monolithic Integrated Circuit Mixer Apparatus" by Brian J. Clifton, Gary D. Alley and Ralph A. Murphy, having Ser. No. 343,034 filed on Jan. 27, 1982 there was described therein a double sideband mixer apparatus which is significantly different from either image enhanced single-sideband mixer or the optimixed double-sideband mixer that is the subject of this application. The mixer which is described in the above application was limited in performance by the effects of an undesirable surface wave resonance that detracted from the mixer's operation. The present invention utilizes a dielectric-induced surface wave resonance to enhance both single and double sideband operation.